1. Field of the Invention
The present invention relates to a liquid crystal display element that displays an image with use of liquid crystals sandwiched between substrates.
2. Description of the Related Art
As shown in FIGS. 12(a) and 12(b), a liquid crystal display element 101 constituting a conventional panel includes (i) two substrates (a lower substrate 121 and an upper substrate 123) divided into a display region a and a non-display region b and (ii) first columnar spacers 125, disposed between the substrates both in the display region a and the non-display region b, which make contact with the two substrates. Sandwiched between the two substrates are liquid crystals 126, and the liquid crystals 126 are sealed in with sealing material 122.
A liquid crystal panel manufactured with the use of flexible substrates such as plastic substrates needs columnar spacers in order to obtain a predetermined cell gap, uniformity, and resistance to pressure. Flexible substrates may have warpage or undulation; therefore, when such substrates are joined to each other, it is necessary to harden a seal with the substrates made sufficiently flat under pressure. For this reason, highly pressure-resistant columnar spacers are used instead of conventionally commonly-used spherical bead spacers. As shown in FIG. 12(a), the columnar spacers of a predetermined size are provided across the whole panel. This makes it possible to obtain a liquid crystal panel that has a uniform cell gap across the whole panel. In cases where a liquid crystal panel manufactured with the use of flexible substrates is subjected to external pressure due to pen-based input or the like, the flexible substrates are, unlike glass substrates, easily deformed at the portion to which force is applied. Therefore, it is preferable that the columnar spacers withstand external pressure and have high rigidity or high elasticity.
Conventional panels are disclosed in Patent Documents 1 to 6 listed below.
Patent Document 1: Japanese Unexamined Patent Application No. 96626/1989 (Tokukaihei 1-96626; published on Jan. 14, 1989)
Patent Document 2: Japanese Unexamined Patent Application No. 338503/2000 (Tokukai 2000-338503; published on Dec. 8, 2000)
Patent Document 3: Japanese Unexamined Patent Application No. 62038/2004 (Tokukai 2004-62038; published on Feb. 26, 2004)
Patent Document 4: Japanese Unexamined Patent Application No. 279998/2003 (Tokukai 2003-279998; published on Oct. 2, 2003)
Patent Document 5: Japanese Unexamined Patent Application No. 212741/2004 (Tokukai 2004-212741; published on Jul. 29, 2004)
Patent Document 6: Japanese Unexamined Patent Application No. 212742/2004 (Tokukai 2004-212742; published on Jul. 29, 2004)
However, such a panel has another problem in terms of reliability. In cases where the panel is left at low temperatures, the volume of the liquid crystals is reduced. However, the substrates between which the cell gap is controlled with the columnar spacers cannot be deformed. Therefore, the capacity of the panel cannot keep pace with a change in the volume of the liquid crystals. This causes generation of regions free of liquid crystals (such regions being hereinafter referred to as “vacuum bubbles”). Further, the same is equally true of a case where the panel is subjected to large external force. For example, in cases where one of the substrates is subjected to force that pulls the substrate away from the liquid crystals (outward), the substrate expands and contracts to such an extent that the capacity of the panel is increased. However, the volume of the liquid crystals cannot keep pace, with the result that vacuum bubbles are generated.
According to Patent Document 1 (Japanese Unexamined Patent Application No. 96626/1989 (Tokukaihei 1-96626; published on Jan. 14, 1989)) (i) spherical spacers having high rigidity and (ii) spherical spacers having low rigidity and a small modulus of elasticity are used. The spherical spacers (ii) have a larger diameter, and are more easily deformed, than the spherical spacers (i). With this, the capacity of the panel can keep pace with a change caused in the volume of the liquid crystals in accordance with a change of temperature.
However, in such a structure, the cell gap in the display region is easily changed due to external pressure or the like. This increases the likelihood of a display defect.
According to Patent Document 2 (Japanese Unexamined Patent Application No. 338503/2000 (Tokukai 2000-338503; published on Dec. 8, 2000)), a spacer body constituted by a plurality of columnar spacers of different heights is provided. The spacer body has a high portion that is easily deformed. This makes it possible to obtain a highly pressure-resistant liquid crystal display element.
However, in such a structure, the cell gap is determined when the high portion of the spacer body has been crushed at the time of joining the substrates to each other. Therefore, the cell gap in the display region, where the capacity of the panel cannot keep pace with a change caused in the volume of the liquid crystals in accordance with a subsequent change of temperature, is easily changed due to external pressure or the like. This increases the likelihood of a display defect.
According to Patent Document 3 (Japanese Unexamined Patent Application No. 62038/2004 (Tokukai 2004-62038; published on Feb. 26, 2004)), no spacers are provided in the non-display region.
However, the use of flexible substrates in this structure causes such a problem that the formation of a vacuum in the panel for the purpose of injecting liquid crystals into the space between the substrates causes the cell gap to collapse, with the result that the liquid crystals are not injected into the space between the substrates.
According to Patent Document 4 (Japanese Unexamined Patent Application No. 279998/2003 (Tokukai 2003-279998; published on Oct. 2, 2003)), the display region and the non-display region have different densities of distribution of columnar spacers.
In this structure, the size of the spacers provided in the display region is equal to the size of the spacers provided in the non-display region; that is, the contact area by which each spacer provided in the display region makes contact with the substrates is equal to the contact area by which each spacer provided in the non-display region makes contact with the substrates, and the display region and the non-display region have different densities of distribution of spacers. Because all the spacers have the same rigidity, the spacer portion is not deformed even under external pressure. Because the spacers are subjected to high pressure, the surfaces of the substrates are damaged in cases where the spacers are harder than the surfaces of the substrates and the spacers are crushed in cases where the surfaces of the substrates are harder than the spacers.
According to Patent Document 5 (Japanese Unexamined Patent Application No. 212741/2004 (Tokukai 2004-212741; published on Jul. 29, 2004)), the columnar spacers provided in the non-display region is lower by 0 to 0.8 μm in height than those provided in the display region.
Because the substrate provided with the columnar spacers has a thick protective layer in the non-display region, the level of an upper surface of each of the columnar spacers provided in the non-display region is made higher than the level of an upper surface of each of the columnar spacers provided in the display region. Therefore, when the height of the spacers provided in the display region is equal to the height of the spacers provided in the non-display region, the substrates are deformed when the substrates are joined to each other. The columnar spacers provided in the non-display region are reduced in height for the purpose of preventing the substrates from being deformed. According to this arrangement, the substrates cannot be deformed in accordance with a change in the volume of the liquid crystals for the following reasons (1) to (3):
(1) The columnar spacers provided in the non-display region and the columnar spacers provided in the display region are in contact with the substrates at a point of time where the substrates are joined to each other;
(2) The columnar spacers provided in the non-display region have the same rigidity as do those provided in the display region; and
(3) It cannot be expected that the non-display region is arranged so as to be easily deformed than the display region (A black matrix usually has a protective layer with high rigidity).
According to Patent Document 6 (Japanese Unexamined Patent Application No. 212742/2004 (Tokukai 2004-212742; published on Jul. 29, 2004)), the area of an upper surface of each of the columnar spacers provided in the non-display region is made smaller than the area of an upper surface of each of the columnar spacers provided in the display region. Because the substrate provided with columnar spacers has a thick protective layer in the non-display region, the level of an upper surface of each of the columnar spacers provided in the non-display region is made higher than the level of an upper surface of each of the columnar spacers provided in the display region. When this substrate is joined to another substrate, the columnar spacers provided in the non-display region are deformed, with the result that a uniform cell gap is obtained.
The columnar spacers provided in the non-display region are not further deformed, and therefore cannot keep pace with a change in the volume of the liquid crystals or the like.